diff --git a/xkb/xkb.c b/xkb/xkb.c
index 5f6420f9e..6e718df8b 100644
--- a/xkb/xkb.c
+++ b/xkb/xkb.c
@@ -2990,6 +2990,8 @@ _XkbSetCompatMap(ClientPtr client, DeviceIntPtr dev,
         XkbSymInterpretPtr sym;
         unsigned int skipped = 0;
 
+        if ((unsigned) (req->firstSI + req->nSI) > USHRT_MAX)
+            return BadValue;
         if ((unsigned) (req->firstSI + req->nSI) > compat->size_si) {
             compat->num_si = compat->size_si = req->firstSI + req->nSI;
             compat->sym_interpret = reallocarray(compat->sym_interpret,